Material having varying birefringence and method of manufacture



350-406 SR Search KOOM 0R 2 v ZQAQ (L? Q J p 16, 1941- a H. LAND2,256,093

MATERIAL HAVING VARYING BIREFRINGENCE AND METHOD OF MANUFACTURE FiledJuly 14, 1958 N ,4

SHEET HflV/AG PEEDEIERH/NED BIZEFE/NGENCE @259 Ham/q o/rrsemr PEEDETEfiM/IVED BlEEFE/NCjE/VCE Patentetl Sept. 16, 1941 Search Rom MATERIALHAVING VABYING BIREFRIN- GENCE AND METHOD OF MANUFACTURE Edwin E. Land,Boston, Masa, minor, by memo assignments, to Polaroid Corporation,Dover, Del., a corporation of Delaware Application July 14, 1938, SerialNo. 219,268

'7 Claims. (Cl. 40-130) This invention relates to a new and improvedmaterial having varying birefringence and to the method of manufacturethereof.

An object of the invention is to provide a sheet or film of birefringentmaterial having predetermined areas thereof possessing a differentbirefringence from that of the remainder of the sheet or film.

Further objects of the invention are to provide a material of thecharacter described comprising birefringent plastic material, such forexample as ethyl cellulose; to provide a material of the characterdescribed wherein the areas of different birefringence comprise indicia,designs,letters, numerals or the like, such for example as halftoneprints or reproductions from photographs and the like; to provide amaterial of the character described wherein the areas of differentbirefringence constitute a unitary, indivisible structure with theremainder of the sheet or film; and to provide a material of thecharacter described wherein substantially the only difference betweenthe sheet as a whole and the said areas of different birefringence is inthe optical properties of the said sheet and said areas.

Other objects of the invention are to provide a method for altering thebirefringence of predetermined areas of a sheet of birefringent materialwhich comprises softening said areas by applying to the said areas asmall amount of a solvent, or a plasticizer, or a swelling agent forsaid sheet, or heating said areas, or embossing said areas; to provide asheet or similar birefringent article for use in the process of theinvention to which there may have been imparted a predeterminedbirefringence, and to alter that birefringence in the manner described.

Other objects of the invention will in part be obvious and will in partappear hereinafter.

The invention accordingly comprises the several steps and the relationof one or more of such steps with respect to each of the others, and theproduct possessing the features, properties, and the relation ofelements which are exemplified in the following detailed disclosure, andthe scope of the application of which will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention,reference should be had to the following detailed description taken inconnection with the accompanying drawing, in which Fig. 1 representssomewhat diagrammatically and in perspective the product and process ofthe invention, and Fig. 2 represents similarly a modification of theinvention.

Various of the sheet-like or film-like forms of plastics commerciallyavailable possess birefringence, in many cases to a high degree. Thisarises from the manner in which the sheets or films are formed, and maybe due to an orientation of the micells entering into the structure ofthe sheets.

This invention contemplates the use of such of the commerciallyavailable birefringent materials as may be adapted to the processhereinafter described. In addition, it is intended that otherbirefringent materials may be employed. It is, for example, possible toalter the birefringence of a sheet of plastic material, such for exampleas a sheet of ethyl cellulose, with or without a plasticizer content, bysubjecting the sheet to heat, so that the sheet becomes soft, and thenstretching the sheet so as to impart to it a predetermined strain. Thesheet is then permitted to cool while the strain is maintained and, uponcooling, the sheet is found to possess a high degree of birefringence.This birefringence may be controlled by controlling the temperature anddegree of extension to which the sheet is subjected. It is also possibleto impart a similarly controlled, predetermined birefringence to plasticmaterials by softening them through the application of a plasticizer ora solvent or swelling agent and then extending them and main tainingthem under strain while the plasticizer or solvent volatilizes or thesheet hardens to an extent sumcient to permit the sheet to retain itsshape and form. It is also possible to impart a high degree ofbirefringence to plastic material by extruding the material from anorifice or die under such conditions that a high degree of micellularorientation is obtained.

Throughout this specification and the claims the term birefringenceshall be understood as meaning retardation per unit thickness. Forexample a sheet of material having a thickness of .010 of an inch, mayfunction as a half-wave retardation plate. Another sheet of material ofthe same thickness may function as a full wave retardation plate. Thesecond sheet is understood to have a higher birefringence than the firstsheet. Throughout this specification and the claims, reference to analteration in the birefringence in the sheet of material is to beunderstood as meaning an alteration in its retardation propertieswithout appreciable alteration in its thickness. By this is meant thatafter a predetermined desired birefringence has been imparted to theplastic sheet, as for example in the manner described, furtheralteration in the bireravfringence of predetermined portions or areas ofthe sheet in the manner hereinafter to be described is to be understoodas being accomplished without substantial alteration in the thickness ofthe sheet including the areas treated to alter their birefringence. Theaction of altering the birefringence of the sheet is intended to includetreatment whereby the degree of molecular orientation of the sheet ischanged. Speaking generally; where a sheet of material employed in thepresent invention possesses a high birefringence, its molecules will besubstantially oriented in a predetermined direction and if thebirefringence of certain portions or areas of that sheet is reduced asfor example in the manner hereinafter described the reduction inbirefringence will generally be accompanied by a disorientation of themolecular structure of the sheet. It should be understood, however, thatin the processes described in connection with the alteration of thebirefringence of the sheet of material treated as a whole, and moreparticularly where the birefringencesef the sheet is predeterminedlyincreased by, for example, heating the sheet and then stretching it toeffect substantial molecular orientation of the sheet, the thickness ofthe sheet may be decreased. Thus a sheet which was substantiallyisotropic or a low fractional wave retardation sheet such for example asa sixteenth-wave retardation sheet may be altered so as to become forexample a full wave or a wave and a half retardation sheet of even lessthickness than the original sheet. It will be obvious that under suchconditions the birefringence of-the sheet, which is to be understood asbeing its retardation per unit thickness, will have been very greatlyincreased.

Any of the processes herein described or any other process which impartsa predetermined desired birefringence to the transparent or translucentplastic or similar material employed in this invention is adapted foruse in connection with the preparation of the birefringent element.Suitable materials adapted to one or more of the preceding processes andto the process of the present invention are cellulosic materials suchfor example as cellulose nitrate, cellulose acetate, cellulose acetatecompounds, ethyl cellulose and the like; vinyl compounds such forexample as polymerized vinyl chloride, polymerized vinyl acetate andmixtures thereof, such as the material sold under the trade nameVinylite, and other similar compounds, such for example as polystyreneresins, and other resinous or plastic materials.

In the drawing, Ill represents generally a sheet or block or filmpossessing a predetermined desired birefringence. This inventioncontemplates a method of altering the birefringence of such an elementover predetermined areas thereof by softening said areas by applyingthereto a solvent, or a plasticizer or a swelling agent, for thebirefringent material, or by heating said areas, or embossing them. Theeffect of the application of such a solvent or plasticizer or heat orpressure is apparently to cause a disorientation, to some extent atleast, of the micells of the sheet or film.

In Fig. 1, l2 indicates an area to which a solvent has been applied, asfor example by the brush II. The degree of alteration of thebirefringence of the sheet In may be controlled by controlling thequantity of solvent applied to the sheet. If, for example, the sheet berelatively thick and only a small amount of solvent be applied to theareas 12, then the orientation of the micells of the sheet may bedestroyed only adjacent the surface to which the solvent has beenapplied. If more solvent be applied so that the use of heat. In thelatter case, as shown in Fig. 2, a heated element such as a rod 24 orplate bearing the desired design may be held against the birefringentsheet Ill until the desired alteration in birefringence has beeneffected, as for example in the area l2.

Solvents, plasticizers and swelling agents have been mentioned assuitable for use in the proces of the present invention. Each acts torelease the strain in the birefringent sheet throughout the area ofapplication of the solvent, plasticizer, or swelling agent. The solventacts to release the strain by forming a solution of the materialcomprising the sheet. The plasticizers act to release the strainbysoftening the material of the birefringent sheet, as do the swellingagents.

It will be apparent that various designs, indicia, numerals, letters,etc. may be formed on the sheet by painting with a solvent, or byapplying a. plasticizer or swelling agent or heat. The solvent itselfmay be colorless, as may the sheet, and it may be desirable to performthe painting operations in polarized light with the sheet positioned sothat its principal optical directions are at predetermined angles to thedirection of vibration of the polarized beam in which the sheet isviewed. Under these circumstances, the birefringence of the sheet itselfmay impart a predetermined color characteristic to the transmitted beam,and the application of the solvent, or swelling agent, or heat, orplasticizer, to the sheet with the corresponding alteration in thebirefringence of predetermined areas thereof re-' sults in an alterationin'the color characteristics of the beam transmitted through thoseareas. In this manner the worker, such for example as the artist who isemploying the process, may see the effect of the application on thebirefringent sheet as he works, and he may control the degree ofalteration in the birefringence of the sheet 10.

The finished product comprises a unitary sheet of plastic material,different areas thereof possessing difierent birefringence, but in allother respects the sheet may be identical over all areas thereof for,with the evaporation of the solvent, the other physical properties ofthe area to which it has been applied may revert substantially to thoseof the sheet itself. This will not be the case where varyingbirefringence has been imparted to the sheet by pressure or embossing.

In one form of the invention, the areas of the birefringent materialacted upon may, together, form a half-tone reproduction. Pictures in thenature of photographic reproductions may be printed in solvent on thesheet and these may be vividly apparent in varying colors when the sheetis viewed in polarized light.

The product of the present invention is of great use in advertisingdisplay devices employing a polarizer and an analyzer with abirefringent sheet therebetween in connection with the projection ofbeams of light possessing varied color characteristics. It may also beuseful in connection with scientific instruments of various typesemploying polarized light in connection with the study of photoelasticeffects and the like.

It will be understood that any solvent of the plastic sheet may beapplied in the process of the invention, as may any plasticizer orswelling agent which sufficiently softens the area to which it isapplied to alter the birefringence thereof.

Since certain changes in carrying out the above process, and certainmodifications in the article which embody the invention may be madewithout departing from its scope, it is intended that all mattercontained in the above description or shown in the accompanying drawingshall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed, and all statements of the scope of the invention which, as amatter of language, might be said to fall therebetween.

Having described my invention, what I claim as new and desire to secureby Letters Patent is:

1. A process for the formation of a design in plastic material having apredetermined birefringence caused by the orientation of the micellularstructure of the said plastic, which design is visible in polarizedlight, comprising heating only the areas of said material forming thesaid design to alter the birefringence thereof by altering themicellular orientation of the plastic material forming the said areas.

2. A process for the formation of a design in plastic materialcomprising imparting to the said material a predetermined birefringenceby causing a predetermined orientation of the micellular structure ofthe said plastic, and altering the micellular orientation of such areasonly of the said plastic material as comprise the said design bysoftening said areas by applying thereto a material from the classconsisting of the solvents, plasticizers and swelling agents for saidSearch R00 plastic material, whereby the birefringence of said areas isaltered.

3. A process comprising applying to predetermined areas of abirefringent plastic a solvent for said plastic to an extent that thebirefringence of said areas is altered without substantial alteration inthe other physical properties of said plastic.

4. As a new article of manufacture, a sheet of birefringent plastic,predetermined areas of said plastic having a predeterminedbirefringence, and other predetermined areas of said plastic having abirefringence differing predeterminedly therefrom, all of said areasbeing of substantially the same thickness, the corresponding principaloptical directions of all said areas being substantially parallel.

5. As a new articleof manufacture, a sheet of birefringent plastic,predetermined areas of said plastic having a predeterminedbirefringence, and other predetermined areas of said plastic having abirefringence differing therefrom, said difference in birefringencebeing due to differences in the degree of micellular orientation withinsaid areas.

6. As a new article of manufacture, a sheet of birefringent plastic,predetermined areas of said plastic having a predeterminedbirefringence, and other predetermined areas of said plastic having abirefringence differing therefrom, said areas of differing birefringencebeing arranged to form a predetermined design when said sheet is viewedin polarized light, the corresponding principal optical directions ofall said areas being substantially parallel.

'7. A sheet of birefringent plastic material having predetermined areasof different birefringence from that of the remainder of said sheet, thedegree of micellular orientation of said areas differing from that ofthe remainder of said sheet, said areas being of such size and shape andbeing so positioned as to impart to a beam of polarized lighttransmitted by said sheet predetermined differing color characteristicsforming a design in the nature of a half-tone print.

EDWIN H. LAND.

